Gas laser apparatus for lithography

ABSTRACT

The present invention relates to a long pulse gas laser apparatus for lithography further improved in the laser oscillation efficiency and stability increased by addition of xenon gas. A gas laser apparatus for lithography has a pair of discharge electrodes  2  provided in a laser chamber  1  and emits laser beam having a laser pulse width (T is ) of not less than 40 ns by exciting a laser gas sealed in the laser chamber  1  by electric discharge between the pair of discharge electrodes  2,  the laser gas containing xenon in an amount not less than 2 ppm and not more than 100 ppm in partial pressure ratio. The laser gas has been heated at least when the laser beam is emitted.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to gas laser apparatus forlithography and, more particularly, to gas laser apparatus forlithography, e.g. KrF excimer laser apparatus, ArF excimer laserapparatus, or fluorine laser apparatus

[0002] With the achievement of small, fine and high-integrationsemiconductor integrated circuits, it has been demanded that exposuresystems for the manufacture of such highly integrated circuits beimproved in resolution. Under these circumstances, the wavelength ofexposure light emitted from light sources for lithography is becomingshorter, and gas laser apparatus emitting light shorter in wavelengththan light emitted from conventional mercury lamps are used as lightsources for semiconductor lithography. At present, KrF excimer laserapparatus that emit ultraviolet radiation of wavelength 248 nm are usedas the gas laser apparatus for lithography. In addition, ArF excimerlaser apparatus emitting ultraviolet radiation of wavelength 193 nm andfluorine (F₂) laser apparatus emitting ultraviolet radiation ofwavelength 157 nm are promising as next-generation light sources forsemiconductor lithography.

[0003] In the KrF excimer laser apparatus, a mixed gas of fluorine (F₂)gas, krypton (Kr) gas and a rare gas as a buffer gas, e.g. neon, is usedas a laser gas, which is a laser medium. In the ArF excimer laserapparatus, a mixed gas of fluorine (F₂) gas, argon (Ar) gas and a raregas as a buffer gas, e.g. neon, is used as a laser gas. In the fluorinelaser apparatus, a mixed gas of fluorine (F₂) gas and a rare gas as abuffer gas, e.g. helium (He) or/and neon (Ne), is used as a laser gas.In these apparatus, the laser gas as a laser medium, which is in theform of a mixed gas, is sealed in a laser chamber under several hundredkPa and excited by generating an electric discharge in the laserchamber.

[0004] More specifically, in the laser chamber, a pair of main dischargeelectrodes for exciting the laser gas are disposed to face each other ata predetermined distance in a direction perpendicular to the laseroscillation direction. A high-voltage pulse is applied between the pairof main discharge electrodes. When the voltage across the main dischargeelectrodes reaches a certain value (breakdown voltage), a dielectricbreakdown occurs in the laser gas between the main discharge electrodes,and thus a main discharge starts. The laser medium is excited by themain discharge.

[0005] Accordingly, such gas laser apparatus for lithography performpulsed laser oscillation by repetition of the main discharge and emitpulsed laser beam. At the present state of the art, the laser pulserepetition rate is 2 kHz or higher.

[0006] Gas laser apparatus for lithography are characterized in thatbecause the oscillation pulse width (T_(is)) is usually about 20 ns, thepeak power of the output light is large, and that because the wavelengthof the output light is short, the photon energy is high. Here, T_(is) isused as the laser oscillation pulse width.

[0007] Assuming that the deterioration of optical elements is caused bythe two-photon absorption of laser light, the damage to the opticalsystem is known to be in inverse proportion to T_(is) under the samelaser energy conditions. T_(is) is defined by

T _(is) =[ƒP(t)dt] ² /ƒP(t)² dt  (1)

[0008] where P(t) is the laser intensity depending upon time t.

[0009] Therefore, it is demanded that the laser pulse width T_(is) bestretched to achieve a longer pulse width as one method for reducing thedamage to an optical system mounted in an exposure system. Theachievement of a longer pulse width is also demanded from the followingpoints of view.

[0010] In a projection exposure system, an image of a mask provided witha circuit pattern or the like is projected through a projection lensonto a work, e.g. a wafer, coated with a photoresist. The resolution Rof the projected image and the depth of focus DOF are expressed by

R=k ₁ λ/NA  (2)

DOF=k ₂λ/(NA)²  (3)

[0011] where k₁ and k₂ are coefficients reflecting the characteristicsof the resist and so forth; λ is the wavelength of exposure lightemitted from a light source for lithography; and NA is a numericalaperture.

[0012] To improve the resolution R, the wavelength of exposure light isreduced, and the NA is increased, as will be clear from Eq. (2).However, the depth of focus DOF decreases correspondingly, as shown byEq. (3). Consequently, the influence of chromatic aberration increases.Therefore, it is necessary to further narrow the spectral linewidth ofexposure light. In other words, it is demanded that the spectrallinewidth of laser beam emitted from the gas laser apparatus forlithography be further narrowed.

[0013] It is stated in Proc. SPIE Vol. 3679.(1999) 1030-1037 thataccording as the laser pulse width increases, the spectral linewidth oflaser beam narrows. This was actually proved by an experiment conductedby the present inventors. In other words, it is demanded in order toimprove the resolution R that the spectral linewidth of laser beam befurther narrowed. For this purpose, it is essential to further stretchthe pulse width of laser beam.

[0014] In regard to the above-described gas laser apparatus forlithography, a technique has heretofore been proposed whereby apredetermined amount of a rare gas (e.g. xenon: Xe) different in kindfrom the rare gas in the laser gas is added to the laser gas to improveenergy stability between the pulses of emitted laser beam and toincrease the laser output.

[0015] More specifically, Japanese Gazette Containing the U.S. Pat. No.3,046,955 states that less than 100 ppm of an additional gas (e.g. lessthan 10 ppm of oxygen and a certain amount of a rare gas (Xe or thelike) heavier than the rare gas in the laser gas) is added to the lasergas sealed in the laser chamber of a KrF excimer laser apparatus or anArF excimer laser apparatus to improve the energy stability.

[0016] Further, it is stated in Japanese Patent Application UnexaminedPublication (KOKAI) Nos. 2000-261074, 2000-261075, 2000-261082 and2000-294856 that if a predetermined amount of xenon gas is added to alaser gas, burst characteristics and spike characteristics are improved.That is, in a burst operation in which a continuous pulse oscillatingoperation for a predetermined period of time and oscillation suspensionfor a predetermined period of time are repeated, it is possible toimprove burst characteristics in which the energy is high at thebeginning and gradually reduces thereafter. It is also possible toimprove spike characteristics in which the energy is high at thebeginning of each continuous pulse oscillating operation and graduallyreduces thereafter.

[0017] As stated above, it is known that addition of xenon to theexcimer laser gas allows an improvement in laser performance, e.g.energy stability. To further increase exposure systems in performanceand to increase the lifetime of gas laser apparatus for lithography,however, it is demanded that the gas laser apparatus for lithography beimproved to provide a higher output and to exhibit higher stability, andfurther, the laser pulse width of laser beam emitted therefrom befurther stretched.

[0018] Under the above-described circumstances, the present inventorstook notice of the temperature of the laser gas at the time of addingxenon thereto and obtained laser characteristic data under varioustemperature conditions. As a result, we found that the lasercharacteristics are highly dependent upon the temperature conditions.

SUMMARY OF THE INVENTION

[0019] The present invention was made in view of the above-describedcircumstances. An object of the present invention is to provide a longpulse gas laser apparatus for lithography improved over the prior art inthe effects of the addition of xenon gas.

[0020] A gas laser apparatus for lithography according to the presentinvention provided to attain the above-described object has a pair ofdischarge electrodes for excitation provided in a laser chamber having alaser gas sealed therein and emits laser beam having a laser pulse width(T_(is)) of not less than 40 ns.

[0021] The gas laser apparatus for lithography is characterized in thatthe temperature of the laser gas before addition of xenon is not lessthan 35° C., and xenon is added to the laser gas at not less than 35° C.in an amount not less than 2 ppm and not more than 100 ppm in partialpressure ratio.

[0022] In this case, it is desirable that the temperature of the lasergas before addition of xenon be not less than 40° C., more preferablynot less than 45° C., and xenon be added to the laser gas in an amountnot less than 2 ppm and not more than 100 ppm in partial pressure ratio.

[0023] Another gas laser apparatus for lithography according to thepresent invention has a pair of discharge electrodes for excitationprovided in a laser chamber having a laser gas sealed therein and emitslaser beam having a laser pulse width (T_(is)) of not less than 40 ns.

[0024] The gas laser apparatus for lithography is characterized in thatthe temperature of the laser chamber is raised to not less than 35° C.by heating, and thereafter, a laser gas containing xenon in an amountnot less than 2 ppm and not more than 100 ppm in partial pressure ratiois sealed in the laser chamber.

[0025] In this case, it is desirable that the temperature of the laserchamber be raised to not less than 40° C., more preferably not less than45° C., by heating.

[0026] Still another gas laser apparatus for lithography according tothe present invention has a pair of discharge electrodes for excitationprovided in a laser chamber having a laser gas sealed therein and emitslaser beam having a laser pulse width (T_(is)) of not less than 40 ns.

[0027] The gas laser apparatus for lithography is characterized in thatxenon is added to the laser gas in an amount not less than 2 ppm and notmore than 100 ppm in partial pressure ratio, and the temperature of thelaser gas after the addition of xenon is not less than 35° C.

[0028] In this case, it is desirable that the temperature of the lasergas after the addition of xenon be not less than 40° C., more preferablynot less than 45° C.

[0029] A further gas laser apparatus for lithography according to thepresent invention has a pair of discharge electrodes provided in a laserchamber and emits laser beam having a laser pulse width (T_(is)) of notless than 40 ns by exciting a laser gas sealed in the laser chamber byelectric discharge between the pair of discharge electrodes, the lasergas containing at least xenon in an amount not less than 2 ppm and notmore than 100 ppm in partial pressure ratio.

[0030] The gas laser apparatus for lithography is characterized in thatthe laser gas has been heated at least when the laser beam is emitted.

[0031] In this case, it is desirable that the gas laser apparatus havepreionization means for preionizing the laser gas.

[0032] It is also desirable that the laser gas should have been heatedto not less than 35° C., more preferably not less than 40° C., even morepreferably not less than 45° C., when laser beam is emitted.

[0033] The foregoing gas laser apparatus may be arranged to perform alaser oscillating operation by the first half-cycle of the dischargeoscillating current waveform of one pulse in which the polarity isreversed, together with at least two half-cycles subsequent to the firsthalf-cycle.

[0034] The foregoing gas laser apparatus for lithography according tothe present invention may be a KrF excimer laser apparatus, an ArFexcimer laser apparatus, or a fluorine laser apparatus, for example.

[0035] According to the present invention, in a gas laser apparatus forlithography that emits laser beam having a laser pulse width (T_(is)) ofnot less than 40 ns, the laser gas containing xenon has been heated whenlaser beam is emitted. Therefore, it is possible to increase the outputenergy and the degree of stability in the second half of each laserpulse. Thus, the laser pulse width is further stretched, and a reductionin the peak power and narrowing of the bandwidth are realized. Inaddition, the laser oscillation efficiency is improved. Therefore, thechamber lifetime is extended owing to the reduction in the input energy,which makes laser operating cost reduced. Further, because the energy inthe second half of each laser pulse is stabilized, energy stability perpulse is improved, and exposure dose controllability is also improved.

[0036] Still other objects and advantages of the invention will in partbe obvious and will in part be apparent from the specification.

[0037] The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a diagram showing a structural example of the gas laserapparatus for lithography according to the present invention.

[0039]FIG. 2 is a diagram showing an example of a discharge circuit inthe gas laser apparatus for lithography shown in FIG. 1.

[0040]FIG. 3 is a diagram showing the effect of the addition of xenonand the laser gas temperature on the laser pulse waveform in a casewhere the laser pulse width is not stretched to achieve a longer pulsewidth.

[0041]FIG. 4 is a diagram showing the effect of the addition of xenonand the laser gas temperature on the laser pulse waveform in an ArFexcimer laser apparatus arranged to achieve a longer pulse width.

[0042]FIG. 5 is a diagram showing the effect of the addition of xenonand the laser gas temperature on laser output characteristics withrespect to the laser pulse repetition rate.

[0043]FIG. 6 is a diagram showing the effect of the addition of xenonand the laser gas temperature on stability characteristics with respectto the laser pulse repetition rate.

[0044]FIG. 7 is a diagram showing the effect of the addition of xenonand the laser gas temperature on laser pulse width (T_(is))characteristics with respect to the laser pulse repetition rate.

[0045]FIG. 8 is a diagram showing the change of the laser output withrespect to the concentration of Xe in a laser gas.

[0046]FIG. 9 is a schematic view showing the discharge current waveformand the laser pulse waveform in a case where a gas laser apparatusperforms a laser oscillating operation by three or more half-cycles ofthe discharge oscillating current waveform.

[0047]FIG. 10 is a diagram showing the effect of the sequentialrelationship between the Xe addition timing and the laser gastemperature raising timing on the repetition rate characteristics of thelaser output.

[0048]FIG. 11 is a diagram showing the effect of the sequentialrelationship between the Xe addition timing and the laser gastemperature raising timing on the repetition rate characteristics of thedegree of stability.

[0049]FIG. 12 is a diagram for describing an example of a laser chamberheating mechanism and a temperature control mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] The gas laser apparatus for lithography according to the presentinvention will be described below on the basis of embodiments.

[0051] A structural example of the gas laser apparatus for lithographyis shown in FIG. 1. A laser chamber 1 having windows provided at bothends thereof is filled with a laser gas having a predeterminedcomposition ratio. A pair of main discharge electrodes 2 for excitingthe laser gas are disposed in the laser chamber 1 to face each other ina direction perpendicular to the laser oscillation direction. Ahigh-voltage pulse generator 3 applies a high-voltage pulse between themain discharge electrodes 2 to generate an electric dischargetherebetween, thereby exciting the laser gas to generate fluorescentlight, which induces laser oscillation.

[0052] It should be noted that the laser gas is forced to circulate inthe laser chamber 1 by rotation of a fan 4 provided in the laser chamber1 to remove ionized substances generated by electric discharge from thedischarge space before the subsequent electric discharge in ahigh-repetition rate oscillating operation. By the laser gascirculation, the laser gas between the main discharge electrodes 2 isreplaced with a new gas after the generation of an electric dischargebefore the generation of the subsequent electric discharge. Therefore,the subsequent electric discharge is allowed to be a stable discharge.

[0053] The present inventors improved the laser gas circulatingstructure in the laser chamber 1, the configuration of the fan 4 and soforth to realize a repetition rate of 2 kHz or higher.

[0054] The fluorescent light generated by the electric dischargereciprocates between a line-narrowing module 5 and an output mirror 6while undergoing selection of a predetermined wavelength by aline-narrowing optical system provided in the line-narrowing module 5.In this way, laser oscillation takes place, and the resulting laser beamis taken out from the output mirror 6. It should be noted that theline-narrowing optical system comprises, for example, a beam expandingoptical system, which is formed from one or more prisms, and a Littrowmounting reflection type diffraction grating.

[0055] A part of the laser beam taken out from the output mirror 6 issplit by a beam sampler 7 and led to a waveform detecting means fordetecting the temporal waveform of the laser beam and also to an opticalenergy detecting means 8 for detecting the energy of the laser beam.Waveform data obtained by the waveform detecting means is sent to apulse width calculating means 9. Energy data obtained by the opticalenergy detecting means is sent to an energy stability calculating means10. The pulse width calculating means 9 calculates a laser pulse widthT_(is) according to the above-described Eq. (1) on the basis of thereceived pulse width data. The energy stability calculating means 10calculates the degree of stability o from the received energy data.

[0056] A discharge circuit as shown in FIG. 2 applies a main dischargevoltage between the main discharge electrodes 2 of the above-describedgas laser apparatus and also applies a predischarge voltage betweenelectrodes 11 and 13 of a corona preionization unit 14 through acapacitor C_(c) for preionization. It should be noted that the coronapreionization unit 14 in this example is arranged as follows. Forexample, the first electrode 11 is formed from a circular column-shapedelectrode inserted into a tube 12, one side of which is open. The tube12 is made of a dielectric material, e.g. high-purity alumina ceramics.The second electrode 13 is formed from a rectangular plate-shapedelectrode. The plate-shaped member, which constitutes the secondelectrode 13, is bent in the vicinity of a straight edge thereof. Theedge of the second electrode 13 is placed parallel to and in linecontact with the outer surface of the dielectric tube 12, whichconstitutes the first electrode 11. The position at which the edge ofthe second electrode 13 contacts the outer surface of the dielectrictube 12 is set at a position from which the laser excitation spacebetween the main discharge electrodes 2 is visible.

[0057] The circuit for generating a high-voltage pulse is a two-stagemagnetic pulse compression circuit as shown in FIG. 2, which uses threemagnetic switches SL0, SL1 and SL2 formed from saturable reactors,respectively. The magnetic switch SL0 protects a solid-state switch SW.The first magnetic switch SL1 and the second magnetic switch SL2constitute a two-stage magnetic pulse compression circuit.

[0058] The arrangement and operation of the circuit will be describedbelow with reference to FIG. 2. First, the voltage of a high-voltagepower supply HV is adjusted to a predetermined value, and a maincapacitor C0 is charged through the magnetic switch SL0 and aninductance L1. At this time, the solid-state switch SW is OFF. Uponcompletion of the charging of the main capacitor C0, the solid-stateswitch SW turns ON. At this time, the voltage across the solid-stateswitch SW shifts so as to be applied across the magnetic switch SL0,thereby protecting the solid-state switch SW. When the time integrationvalue of the charging voltage VO across the main capacitor C0, which isapplied across the magnetic switch SL0, reaches a critical valuedetermined by the characteristics of the magnetic switch SL0, themagnetic switch SL0 is saturated to turn ON. Consequently, an electriccurrent flows through a loop formed by the main capacitor C0, themagnetic switch SL0, the solid-state switch SW and a capacitor C1. As aresult, the electric charge stored in the main capacitor C0 istransferred to and stored in the capacitor C1.

[0059] Thereafter, when the time integration value of the voltage V1across the capacitor C1 reaches a critical value determined by thecharacteristics of the magnetic switch SL1, the magnetic switch SL1 issaturated to turn ON. Consequently, an electric current flows through aloop formed by the capacitor C1, a capacitor C2 and the magnetic switchSL2. As a result, the electric charge stored in the capacitor C1 istransferred to and stored in the capacitor C2.

[0060] Thereafter, when the time integration value of the voltage V2across the capacitor C2 reaches a critical value determined by thecharacteristics of the magnetic switch SL2, the magnetic switch SL2 issaturated to turn ON. Consequently, an electric current flows through aloop formed by the capacitor C2, a peaking capacitor Cp and the magneticswitch SL2. As a result, the electric charge stored in the capacitor C2is transferred to and stored in the peaking capacitor Cp.

[0061] As will be clear from the description given in connection withFIG. 2, a corona discharge for preionization occurs at the outerperipheral surface of the dielectric tube 12, starting from the positionat which the second electrode 13 contacts the dielectric tube 12. Morespecifically, as the charging of the peaking capacitor Cp, which isshown in FIG. 2, proceeds, the voltage V3 across the peaking capacitorCp increases. When the voltage V3 reaches a predetermined value, acorona discharge occurs at the surface of the dielectric tube 12 of thecorona preionization unit. The corona discharge causes ultravioletradiation to be generated at the surface of the dielectric tube 12. Theultraviolet radiation preionizes the laser gas flowing between the maindischarge electrodes 2 as a laser medium.

[0062] As the charging of the peaking capacitor Cp further proceeds, thevoltage V3 across the peaking capacitor Cp increases. When the voltageV3 reaches a certain value (breakdown voltage) Vb, an electric breakdownoccurs in the laser gas between the main discharge electrodes 2, andthus a main discharge starts. The laser medium is excited by the maindischarge, and laser beam is generated.

[0063] Thereafter, the voltage across the peaking capacitor Cp lowersrapidly owing to the main discharge, and eventually returns to the statebefore the start of charging.

[0064] The above-described discharging operation is repeated by theswitching operation of the solid-state switch SW, whereby pulsed laseroscillation is performed at a predetermined repetition rate.

[0065] Thus, a combination of the magnetic switch SL1 and the capacitorC1 forms a capacitive transfer circuit constituting a first stage, and acombination of the magnetic switch SL2 and the capacitor C2 forms acapacitive transfer circuit constituting a second stage. By setting theinductance of each capacitive transfer circuit so that the inductancebecomes smaller as the ordinal number of stages increases, a pulsecompression operation is carried out such that the pulse width of anelectric current pulse flowing through each stage narrows successively.Consequently, a strong discharge of short pulse is realized between themain discharge electrodes 2.

[0066] We examined the relationship between the addition of xenon (Xe)to the laser gas and the laser gas temperature in a case where theabove-described gas laser apparatus for lithography is not particularlyarranged to stretch the pulse width as in the case of the conventionalapparatus. FIG. 3 shows an example of a laser pulse waveform in a gaslaser apparatus (ArF excimer laser apparatus) for lithography that isnot particularly arranged to stretch the pulse width, i.e. which has alaser pulse width (T_(is)) of less than 40 nm under the conditions wherethe laser gas does not contain Xe and the laser gas temperature is 24°C. {circle over (1)}, in FIG. 3).

[0067] Three different kinds of curves {circle over (1)}, {circle over(2)} and {circle over (3)} drawn in FIG. 3 are as follows: Curve {circleover (1)} shows a laser pulse waveform under the conditions that thelaser gas does not contain Xe and the laser gas temperature is 24° C.(hereinafter referred to as “conditions {circle over (1)}”); curve{circle over (2)} shows a laser pulse waveform under the conditions thatthe laser gas contains 10 ppm of Xe and the laser gas temperature is 24°C. (hereinafter referred to as “conditions {circle over (2)}”); andcurve {circle over (3)} shows a laser pulse waveform under theconditions that the laser gas contains 10 ppm of Xe and the laser gastemperature is 50° C. (hereinafter referred to as “conditions {circleover (3)}”).

[0068] Under any of the three conditions, the laser pulse width (T_(is))was less than 40 ns.

[0069] The area of the laser pulse waveform is equivalent to the laseroutput. As will be clear from FIG. 3, the laser output is larger underthe conditions where Xe is added to the laser gas (conditions {circleover (2)} and {circle over (3)}) than under the conditions where Xe isnot added to the laser gas (conditions {circle over (1)}). However,there is substantially no difference in the laser pulse waveform, i.e.in the laser output, between the conditions where Xe is added to thelaser gas and the laser gas temperature is low (conditions {circle over(2)}) and the conditions where Xe is added to the laser gas and thelaser gas temperature is high (conditions {circle over (3)}).

[0070] That is, the effects of the addition of Xe similar to those inthe prior art are recognized in an ArF excimer laser apparatus notparticularly arranged to stretch the pulse width, i.e. which has a laserpulse width (T_(is)) of less than 40 ns under the conditions where thelaser gas does not contain Xe and the laser gas temperature is 24° C.However, substantially no effects of raising the temperature arerecognized in such an ArF excimer laser apparatus.

[0071] The present inventors found that, in contrast to the prior art,the laser pulse waveform changes markedly in the second half of thewaveform (i.e. in the second half of the laser oscillation) in an ArFexcimer laser apparatus arranged to stretch the pulse width, i.e. whichhas a laser pulse width (T_(is)) of not less than 40 ns under theconditions where the laser gas does not contain Xe and the laser gastemperature is 50° C., if Xe is added to the laser gas and further thelaser gas temperature is raised. The changes in the laser pulse waveformare shown in FIG. 4.

[0072] Three curves shown in FIG. 4 were obtained under three differentconditions (conditions {circle over (1)}, conditions {circle over (2)}and conditions {circle over (3)} similar to those in the case of FIG. 3.Under any of the three conditions, the laser pulse width (T_(is)) wasnot less than 40 ns.

[0073] As will be clear from FIG. 4, in comparison of the conditions(conditions {circle over (1)}) where the laser gas temperature is 24° C.and Xe is not added to the laser gas with the conditions (conditions{circle over (2)}) where the laser gas temperature is 24° C. and Xe isadded to the laser gas, the laser pulse waveform is more protuberant andthe laser output is larger under the conditions {circle over (2)} thanunder the conditions {circle over (1)} in both the first and secondhalves of the laser pulse waveform.

[0074] The above result is similar to that in the case of the foregoingArF excimer laser apparatus that is not particularly arranged to stretchthe pulse width. This also corresponds to a well-known technique inwhich the laser output is increased by adding a predetermined amount ofXe to the laser gas, as stated for example in Japanese PatentApplication Unexamined Publication (KOKAI) No. 2000-294856.

[0075] On the other hand, in comparison of the conditions (conditions{circle over (2)}) where Xe is added to the laser gas and the laser gastemperature is low (24° C.) with the conditions (conditions {circle over(3)}) where Xe is added to the laser gas and the laser gas temperatureis high (50° C.), there is substantially no change in the laser pulsewaveform with the rise in temperature in the first half of the laserpulse waveform. In the second half of the laser pulse waveform, however,the laser pulse waveform is markedly protuberant under the conditions{circle over (3)} in comparison to that under the conditions {circleover (2)}. Thus, the laser output in the second half of the laser pulsewaveform is larger under the conditions {circle over (3)} than under theconditions {circle over (2)}. Further, as shown in FIG. 6, the stabilityof the laser pulse energy is more improved under the conditions {circleover (3)} than under the conditions {circle over (2)}. It should benoted that the degree of stability a is defined by (standard deviationof each pulse energy)/(average of each pulse energy)×100%.

[0076] As will be clear from FIG. 4, it has become clear that raisingthe laser gas temperature as well as adding xenon (Xe) to the laser gasas in the present invention further improves the effects of increasingthe laser output by adding a predetermined amount of xenon (Xe) to thelaser gas, as has heretofore been known.

[0077] It should be noted that experiments conducted by the presentinventors revealed that the effects of adding xenon (Xe) to the lasergas for lithography and, at the same time, raising the laser gastemperature according to the present invention are remarkable when thelaser pulse width (T_(is)) is not less than 40 ns, and the effectsobtained when the laser pulse width (T_(is)) is less than 40 ns are notso different from those obtained with the prior art. It was alsorevealed that the effects obtained by the present invention areparticularly remarkable when the laser gas temperature is not less than35° C., and the effects become greater as the laser gas temperatureincreases to 40° C. or 45° C., for example. On the other hand, when thelaser gas temperature is less than 35° C., the effects are not sodifferent from those obtained with the prior art.

[0078]FIG. 5 shows laser output characteristics with respect to thelaser pulse repetition rate of a gas laser apparatus for lithography(ArF excimer laser apparatus) having a laser pulse width (T_(is)) of 40ns under three different conditions similar to those in the case of FIG.4: conditions {circle over (1)} where the laser gas does not contain Xeand the laser gas temperature is 24° C.; conditions {circle over (2)}where the laser gas contains 10 ppm of Xe and the laser gas temperatureis 24° C.; and conditions {circle over (3)} where the laser gas contains10 ppm of Xe and the laser gas temperature is 50° C. FIG. 6 showsstability characteristics with respect to the laser pulse repetitionrate of the same gas laser apparatus for lithography under the threeconditions {circle over (1)}, {circle over (2)} and {circle over (3)}.FIG. 7 shows laser pulse width (T_(is)) characteristics with respect tothe laser pulse repetition rate of the same gas laser apparatus forlithography under the three conditions {circle over (1)}, {circle over(2)} and {circle over (3)}.

[0079] As shown in FIG. 5, the laser output obtained under theconditions {circle over (3)} is larger than under the conditions {circleover (1)} and {circle over (2)}. As shown in FIG. 6, the value of thestability σ under the conditions {circle over (3)} is smaller than underthe conditions {circle over (1)} and {circle over (2)}. As shown in FIG.7, the laser pulse width (T_(is)) under the conditions {circle over (3)}is longer than under the conditions {circle over (1)} and {circle over(2)}.

[0080] In other words, as will be clear from FIGS. 5, 6 and 7, it ispossible to improve the laser output characteristics, stabilitycharacteristics and laser pulse width (T_(is)) characteristics withrespect to the laser pulse repetition rate by applying the technique ofthe present invention, wherein xenon (Xe) is added to the laser gas andthe laser gas temperature is raised, to a gas laser apparatus forlithography arranged to stretch the pulse width, thereby increasing thelaser output and improving the stability in the second half of eachlaser pulse, as shown in FIG. 4.

[0081] It is not necessarily clear why the second half of the laserpulse waveform is further increased by adding Xe to the laser gas andmaintaining the laser gas temperature at not less than 35° C. in a gaslaser apparatus for lithography arranged to stretch the laser pulsewidth (T_(is)) to not less than 40 ns. However, the reason may besurmised as follows.

[0082]FIG. 8 shows the change of the laser output with respect to theconcentration of Xe in the laser gas. The laser output increases withthe rise in the concentration of Xe in the laser gas. However, when theXe concentration exceeds a certain level, the laser output reduces. Thereason for this is deemed as follows. In a region where the Xeconcentration is not in excess of the certain level, as theconcentration of Xe in the laser gas increases, the laser output alsoincreases owing to the amount of preionization increasing effectsproduced by the addition of Xe. However, in a region where the Xeconcentration is in excess of the certain level, the loss of ultravioletlight due to absorption by Xe or a Xe compound (the term “ultravioletlight” as used herein means light of wavelength λ of the order of 100 nmused for preionization and light emission at the laser oscillationwavelength) exceeds the increase in the laser output caused by theincrease of the preionization quantity. Consequently, the laser outputreduces. In other words, it is necessary in order to optimize the laseroutput that the concentration of Xe in the laser gas be within a certainconcentration range.

[0083] It is desirable that the amount of xenon (Xe) added to the lasergas be of the order of 2 to 100 ppm. The reason for this is as follows.Japanese Patent Application Unexamined Publication (KOKAI) No.2000-294856 reveals that if the amount of Xe added to the laser gas isless than 2 ppm, the effect of easing the reduction in the laser outputin a high-repetition frequency operation cannot substantially beobtained, whereas if the amount of Xe added to the laser gas is morethan 100 ppm, the laser output itself is reduced. This has also beenconfirmed by experiments conducted by the present inventors.

[0084] Incidentally, because the atom of Xe is larger than other atomsin the laser gas, a part of Xe introduced into the laser chamber in avery small quantity is likely to be adsorbed on the laser chamber wallby van der Waals' forces (intermolecular forces). Xe may be adsorbed ineither of two forms, i.e. Xe alone or a compound composed of Xe and acompound containing F. Therefore, when Xe is added to the laser gas toobtain an improvement in laser output stability and other effects, theconventional practice is to add Xe to the laser gas in excess of therequired quantity by an amount corresponding to a reduction in Xeconcentration due to the adsorption on the laser chamber wall, therebyoptimizing the concentration of Xe in the laser gas.

[0085] If the laser operates in this state, Xe adsorbed on the laserchamber wall is desorbed upon obtaining energy exceeding a certain valueby collision with active chemical species produced in the laser chamberby irradiation with the above-described ultraviolet light or electricdischarge, and the desorbed Xe is released in the discharge space. Apart of the desorbed Xe is adsorbed on the laser chamber wall again. Forthis reason, the concentration of Xe in the discharge space is deemed tobe unstable.

[0086] The laser output and stability in the second half of the laserpulse waveform with a stretched pulse width depend strongly on thecapability of sustaining stable electric discharge. One of factors thatexert an influence upon the stable discharge sustainability is thenumber of preionized electrons increased by the addition of Xe to thelaser gas. Accordingly, it is thought that the instability of theconcentration of Xe present in the discharge space causes the electricdischarge to become unstable, and the instability of the electricdischarge causes the laser output to become unstable in the second halfof each laser pulse.

[0087] For the reasons stated above, the adsorption of Xe added to thelaser gas on the laser chamber wall is suppressed by previously raisingthe laser gas temperature to raise the temperature of the laser chamberwall surface in advance. Thus, it is possible to minimize the change ofthe Xe concentration by the lasing operation, and stable electricdischarge is sustained even in the second half of the laser oscillationto generate laser light having a stretched pulse width. That is, it isdeemed that in the case of the conditions {circle over (3)} theabove-described mechanism leads to an increase in the output and animprovement in stability in the second half of each laser pulse with astretched pulse width.

[0088] Meanwhile, in the laser apparatus that is not particularlyarranged to stretch the pulse width, the laser oscillation has alreadystopped in a time region corresponding to the above-described secondhalf of each laser pulse. Therefore, even if the laser gas temperatureis raised after the addition of Xe to the laser gas, there is nodifference in the laser oscillation characteristics obtained when thelaser gas temperature is raised and when it is not.

[0089] Although in the foregoing the ArF excimer laser apparatus is usedas a gas laser apparatus for lithography, it is apparent that theabove-described mechanism also holds true in KrF excimer laser apparatusand fluorine laser apparatus. Accordingly, the above-described effectscan also be obtained in KrF excimer laser apparatus and fluorine laserapparatus.

[0090] Incidentally, the present inventors developed a method wherebythe laser pulse width (T_(is)) of laser beam emitted from a gas laserapparatus for lithography is stretched to 40 ns or more. That is, thepresent inventors realized the stretch of the pulse width by arrangingthe gas laser apparatus for lithography shown in FIGS. 1 and 2 so thatit performs a laser oscillating operation by the first half-cycle of thedischarge oscillating current waveform of one pulse in which thepolarity is reversed, together with at least two half-cycles subsequentto the first half-cycle.

[0091]FIG. 9 is a schematic view showing the discharge current waveformand the laser pulse waveform. The present inventors realized the stretchof the pulse width of laser light emitted from the gas laser apparatusfor lithography by arranging the gas laser apparatus so that laseroscillation is continuously performed by the discharge current inhalf-cycles after the half-cycle I in FIG. 9, i.e. at least thehalf-cycles II and III, in addition to the half-cycle I.

[0092] More specifically, in order to allow laser oscillation tocontinue even after the first half-cycle of the oscillating currentflowing between the main discharge electrodes 2 in FIG. 2, after whichthe polarity of the oscillating current is reversed, circuit constantsare determined so that the peak value of the electric current isincreased and, at the same time, the period of the oscillating currentafter the first half-cycle is shortened. That is, to increase the peakvalue of the oscillating current, the rise time of the voltage appliedbetween the main discharge electrodes 2 is shortened so that thedischarge starting voltage Vb becomes high. To shorten the period of theoscillating current, the loop (discharge current circuit) formed by thepeaking capacitor Cp and the main discharge electrodes 2 in the excitingcircuit shown in FIG. 2 is constructed so that the stray inductance inthe loop is minimized. Further, the composition ratio, pressure and soforth of the laser gas in the laser chamber 1 are adjusted.

[0093] By the above-described mechanism, when the amount of Xe in thedischarge part becomes unstable, electric discharge becomes spatiallynonuniform owing to the concentration of electric discharge or the likewhen the polarity of the discharge current is reversed (I→II in FIG. 9),and energy that can be taken out as a laser output after the firsthalf-cycle I decreases. Accordingly, although the stretch of the pulsewidth can be attained with the prior art, the energy in the second halfof each laser pulse cannot satisfactorily be taken out as laser beam.Consequently, the output in the second half of each laser pulse is low,and the laser pulse width (T_(is)) is small.

[0094] The following is a description of the operation of the presentinvention in the gas laser apparatus for lithography arranged to stretchthe pulse width as stated above. According to the foregoing mechanism,it is conceived that if the laser gas temperature is previously raisedto raise the laser chamber wall surface temperature in advance, theadsorption of Xe added to the laser gas on the laser chamber wall issuppressed, and the change in the Xe concentration caused by the lasingoperation can be minimized. Consequently, when the polarity of thedischarge current is reversed (i.e. at the time of shifting from thehalf-cycle I to the half-cycle II of the oscillating current waveformshown in FIG. 9), the concentration of electric discharge or the like isless likely to occur than in a case where the laser gas temperature isnot raised. Accordingly, uniform discharge is sustained even morefavorably after the half-cycle I. Further, stable discharge is readilysustained even after the half-cycle II of the oscillating current. Inother words, it is possible to increase the laser output and stabilityin the second half of each laser pulse.

[0095] Thus, the present invention provides a means whereby the laseroutput and stability can be further improved and the laser pulse width(T_(is)) can be further stretched by adding Xe to the laser gas andraising the laser gas temperature in the gas laser apparatus forlithography arranged to stretch the pulse width. It should be noted thatthe means according to the present invention is not necessarily limitedin its application to a case where a longer pulse width is achieved byarranging the gas laser apparatus so that it performs a laseroscillating operation by the first half-cycle of the dischargeoscillating current waveform of one pulse in which the polarity isreversed, together with at least two half-cycles subsequent to the firsthalf-cycle. The above-described means is similarly applicable to anycase where the discharge duration time is extended to achieve a longerpulse width, and similar advantageous effects can be obtained.

[0096] Incidentally, we found that the above-described effects of thepresent invention differ in the degree of effectiveness when the lasergas temperature is raised after Xe has been added to the laser gas andwhen Xe is added to the laser gas after the laser gas temperature hasbeen raised. It should be noted that in the former case, the laser gastemperature was raised by heating the laser chamber with a heating means(not shown in FIG. 1). In the latter case also, raising the laser gastemperature before the addition of Xe was effected by heating the laserchamber with the heating means in the same manner as in the former case.It should be noted that in the latter case, Xe may be added after thepreheated laser gas has been sealed in the laser chamber instead ofheating the laser chamber with the heating means.

[0097]FIG. 10 shows the repetition rate characteristic of the laseroutput in a case where the laser gas temperature is raised to 50° C.after 10 ppm of Xe has been added to the laser gas and in a case where10 ppm of Xe is added to the laser gas after the laser gas temperaturehas been raised to 50° C. FIG. 11 shows the repetition ratecharacteristic of the degree of stability in the above-described twocases. It should be noted that the gas laser apparatus for lithographyused in the experiments is an ArF excimer laser apparatus arranged tostretch the pulse width by performing a laser oscillating operation withthe first half-cycle of the discharge oscillating current waveform ofone pulse in which the polarity is reversed, together with at least twohalf-cycles subsequent to the first half-cycle, so that the laser pulsewidth (T_(is)) is 40 ns or more when the Xe has not yet been added tothe laser gas.

[0098] As will be clear from FIGS. 10 and 11, the addition of Xe afterraising the laser gas temperature provides greater effects than theaddition of Xe before raising the laser gas temperature. The reason forthis is not clear but may be as follows.

[0099] In a state where the laser gas temperature is low, thetemperature of the wall in the laser chamber is also low. Therefore, itis deemed that chemical species having particularly large intermolecularforces are adsorbed on the metal or the like constituting the wall. Oneexample of such chemical species is HF (produced by the reaction ofimpurities such as water (H₂O) in the laser chamber with fluorine in thelaser gas), which has a boiling point in the neighborhood of roomtemperature. When a slight amount of Xe is introduced into such anenvironment, a part of the introduced Xe combines weakly with thechemical species adsorbed on the laser chamber wall and is captured onthe laser chamber wall. It is deemed that the compound composed of Xeand other chemical species does not completely release Xe into the lasergas even if the temperature rises (a part of the Xe will be released).Therefore, the increase in the laser output weakens for the same reasonsas the foregoing mechanism.

[0100] That is, a part of Xe added to the laser gas that did not form acompound which does not completely release Xe into the laser gas even ifthe temperature is raised as stated above is hardly adsorbed on ordesorbed from the laser chamber wall after the laser gas temperature hasbeen raised. Therefore, the Xe concentration becomes stabilized.However, Xe forming the above-described compound causes the Xeconcentration to become unstable by desorption from the laser chamberwall. Accordingly, the instability of the Xe concentration remains,although the concentration of Xe in the laser gas is stable incomparison to a case where the laser gas temperature is not raised.

[0101] On the other hand, in a state where the laser gas temperature ishigh, the temperature of the wall in the laser chamber is also high.Therefore, it is deemed that chemical species adsorbed on the laserchamber wall at low temperature have also been desorbed. If a slightamount of Xe is introduced into such an environment, collisions betweenXe and chemical species, which were present on the laser chamber wall atlow temperature, take place in a gas phase. In this case, it is deemedimpossible for Xe to form a compound that may be adsorbed on the wall.Therefore, there is substantially no Xe that may be desorbed from thewall by electric discharge. Thus, the concentration of Xe in the gasbecomes stabilized.

[0102] For the reasons stated above, it is deemed preferable to add apredetermined amount of Xe in a state where the gas temperature has beenraised in advance.

[0103] Incidentally, the use of a premixed laser gas having apredetermined amount of Xe added to a conventional laser gas in advanceis in the mainstream in the present state of the art. Therefore, if thetemperature of the inner wall of the laser chamber has been raised inadvance, it is possible to obtain the same effect as that obtained byadding Xe after the laser gas temperature has been raised.

[0104] The above-described effects can also be provided by bringing aheat source into contact with the chamber to thereby maintain thechamber at an optimum temperature of not less than 35° C. For example,as shown in FIG. 12, a heater 20 as a heat source is placed in contactwith the outer surface of the laser chamber 1. The temperature of thelaser chamber 1 is measured with a temperature sensor 21 installed onthe outer surface of the laser chamber 1. Temperature data thus obtainedis sent to a controller 22. The controller 22 instructs a power supply(not shown) for the heater 20 to control the supply of electric power tothe heater 20 so that the temperature of the laser chamber 1 is at 35°C. or more. The upper limit of the temperature of the laser chamber 1 isappropriately set by taking into consideration the heat resistance ofthe seal portion of the laser chamber 1 and laser components in thevicinity of the laser chamber 1 and so forth.

[0105] It should be noted that the temperature sensor 21 may be disposedon the outer surface of the laser chamber 1 as stated above.Alternatively, the temperature sensor 21 may be disposed in the laserchamber 1 to measure the temperature of the inner wall of the laserchamber 1 directly. In a case where the temperature sensor 21 isdisposed on the outer surface of the laser chamber 1, it is desirablethat the preset temperature be not less than 40° C. or not less than 45°C., for example, because there may be a temperature difference betweenthe outer surface of the laser chamber 1 and the inner wall surfacethereof according to the thermal conductivity of the laser chamber 1.

[0106] Although the gas laser apparatus for lithography according to thepresent invention has been described above on the basis of theembodiments, the present invention is not limited to the foregoingembodiments but can be modified in a variety of ways.

[0107] As will be clear from the foregoing description, according to thepresent invention, in a gas laser apparatus for lithography adapted tostretch the pulse width, the temperature of a laser gas containing aslight amount of Xe is raised, whereby it becomes possible to increasethe laser output and stability in the second half of each laser pulse.Thus, the laser pulse width is further stretched, and a reduction in thepeak power and narrowing of the bandwidth are realized. In addition, thelaser oscillation efficiency is improved. Therefore, the chamberlifetime is extended owing to the reduction in the input energy, and thelaser operating cost is reduced. Further, because the energy in thesecond half of each laser pulse is stabilized, energy stability perpulse is improved, and exposure dose controllability is also improved.The effects are particularly enhanced by adding Xe after the gastemperature has been raised, or by introducing a laser gas containing aslight amount of Xe after the chamber temperature has been raised.

What we claim is:
 1. A gas laser apparatus for lithography having a pairof discharge electrodes for excitation provided in a laser chamberhaving a laser gas sealed therein and emitting laser beam having a laserpulse width (T_(is)) of not less than 40 ns, wherein a temperature ofthe laser gas before addition of xenon is not less than 35° C., andxenon is added to the laser gas at not less than 35° C. in an amount notless than 2 ppm and not more than 100 ppm in partial pressure ratio. 2.A gas laser apparatus for lithography according to claim 1, wherein thetemperature of the laser gas before addition of xenon is not less than40° C., and xenon is added to the laser gas in an amount not less than 2ppm and not more than 100 ppm in partial pressure ratio.
 3. A gas laserapparatus for lithography according to claim 1, wherein the temperatureof the laser gas before addition of xenon is not less than 45° C., andxenon is added to the laser gas in an amount not less than 2 ppm and notmore than 100 ppm in partial pressure ratio.
 4. A gas laser apparatusfor lithography having a pair of discharge electrodes for excitationprovided in a laser chamber having a laser gas sealed therein andemitting laser beam having a laser pulse width (T_(is)) of not less than40 ns, wherein a temperature of the laser chamber is raised to not lessthan 35° C. by heating, and thereafter, a laser gas containing xenon inan amount not less than 2 ppm and not more than 100 ppm in partialpressure ratio is sealed in the laser chamber.
 5. A gas laser apparatusfor lithography according to claim 4, wherein the temperature of thelaser chamber is raised to not less than 40° C. by heating.
 6. A gaslaser apparatus for lithography according to claim 4, wherein thetemperature of the laser chamber is raised to not less than 45° C. byheating.
 7. A gas laser apparatus for lithography having a pair ofdischarge electrodes for excitation provided in a laser chamber having alaser gas sealed therein and emitting laser beam having a laser pulsewidth (T_(is)) of not less than 40 ns, wherein xenon is added to saidlaser gas in an amount not less than 2 ppm and not more than 100 ppm inpartial pressure ratio, and a temperature of the laser gas afteraddition of the xenon is not less than 35° C.
 8. A gas laser apparatusfor lithography according to claim 7, wherein the temperature of thelaser gas after addition of the xenon is not less than 40° C.
 9. A gaslaser apparatus for lithography according to claim 7, wherein thetemperature of the laser gas after addition of the xenon is not lessthan 45° C.
 10. A gas laser apparatus for lithography having a pair ofdischarge electrodes provided in a laser chamber and emitting laser beamhaving a laser pulse width (T_(is)) of not less than 40 ns by exciting alaser gas sealed in said laser chamber by electric discharge betweensaid pair of discharge electrodes, said laser gas containing xenon in anamount not less than 2 ppm and not more than 100 ppm in partial pressureratio, wherein said laser gas has been heated at least when said laserbeam is emitted.
 11. A gas laser apparatus for lithography according toclaim 10, further having preionization means for preionizing said lasergas.
 12. A gas laser apparatus for lithography according to claim 10 or11, wherein said laser gas has been heated to not less than 35° C. whensaid laser beam is emitted.
 13. A gas laser apparatus for lithographyaccording to claim 10 or 11, wherein said laser gas has been heated tonot less than 40° C. when said laser beam is emitted.
 14. A gas laserapparatus for lithography according to claim 10 or 11, wherein saidlaser gas has been heated to not less than 45° C. when said laser beamis emitted.
 15. A gas laser apparatus for lithography according to anyone of claims 1 to 14, which is arranged to perform a laser oscillatingoperation by a first half-cycle of a discharge oscillating currentwaveform of one pulse in which a polarity is reversed, together with atleast two half-cycles subsequent to the first half-cycle.
 16. A gaslaser apparatus for lithography according to any one of claims 1 to 15,which is on e selected from a KrF excimer laser apparatus, an ArFexcimer laser apparatus, and a fluorine laser apparatus.